podman cp: support copying on tmpfs mounts

Traditionally, the path resolution for containers has been resolved on
the *host*; relative to the container's mount point or relative to
specified bind mounts or volumes.

While this works nicely for non-running containers, it poses a problem
for running ones.  In that case, certain kinds of mounts (e.g., tmpfs)
will not resolve correctly.  A tmpfs is held in memory and hence cannot
be resolved relatively to the container's mount point.  A copy operation
will succeed but the data will not show up inside the container.

To support these kinds of mounts, we need to join the *running*
container's mount namespace (and PID namespace) when copying.

Note that this change implies moving the copy and stat logic into
`libpod` since we need to keep the container locked to avoid race
conditions.  The immediate benefit is that all logic is now inside
`libpod`; the code isn't scattered anymore.

Further note that Docker does not support copying to tmpfs mounts.

Tests have been extended to cover *both* path resolutions for running
and created containers.  New tests have been added to exercise the
tmpfs-mount case.

For the record: Some tests could be improved by using `start -a` instead
of a start-exec sequence.  Unfortunately, `start -a` is flaky in the CI
which forced me to use the more expensive start-exec option.

Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/libpod/container_internal.go b/libpod/container_internal.go
index 7e8226de4..a7da7f395 100644
--- a/libpod/container_internal.go
+++ b/libpod/container_internal.go
@@ -2086,6 +2086,10 @@ func (c *Container) setupOCIHooks(ctx context.Context, config *spec.Spec) (map[s
 
 // mount mounts the container's root filesystem
 func (c *Container) mount() (string, error) {
+	if c.state.State == define.ContainerStateRemoving {
+		return "", errors.Wrapf(define.ErrCtrStateInvalid, "cannot mount container %s as it is being removed", c.ID())
+	}
+
 	mountPoint, err := c.runtime.storageService.MountContainerImage(c.ID())
 	if err != nil {
 		return "", errors.Wrapf(err, "error mounting storage for container %s", c.ID())